The reason you assume a gun is loaded isn't because you believe in the spontaneous generation of bullets... it's because that people can be wrong and if you do something with a gun that is predicated on the belief that it's unloaded and you're wrong, the consequences are likely to be horrific and irrevocable. For instance, if you pick up a gun at point at somebody and then because you're sure it's unloaded you pull the trigger just to be funny, no amount of your confidence in your memory is going to bring them back if you're wrong. So the solution is to not do that. Given the huge risk, the tiny benefit, and the odds of being wrong in a given circumstance don't merit it.

But this is just a computer. Picking the "safe" assumption as the default and slavishly adhering to it is not warranted because life and death are not at stake. If I'm wrong, I still have the option to reinstall and start over. If you pick wrong with the gun, there's no edit-undo.

And besides of which, the chances of disinfecting an infected computer are much higher than that of death by bullet-gremlin. It has happened. It might not happen in this case because my computer was highly compromised, but I've seen it happen. Not every bit of malware out there is equally sophisticate or pernicious. Some depend on holes that can be plugged fairly easily.

Really, you've got a perfectly sound and reasonable strategy, but by phrasing it as an absolute, you come off as being much less reasonable.